1. Field of the Invention
The present invention relates to an optical recording medium.
2. Discussion of Background
In recent years, an attention has been drawn to shortening the oscillation wavelength of a laser beam for high density recording, and an optical recording medium has been desired which is useful for recording and reproducing by a laser beam with a wavelength shorter than 780 nm or 830 nm, for example, with a wavelength of 640 nm. Under such circumstances, various recording media are available, but among them, an organic dye type optical recording medium has a merit that it is inexpensive and easy from the viewpoint of its production process.
As an organic dye type optical recording medium, an optical disc (CD-R) of the type which is compatible with CD, has already been practically developed which is operable by a laser beam with a wavelength of 780 nm. On the other hand, various proposals have been made for organic dye-type media for short wavelength application, such as those disclosed in Japanese Unexamined Patent Publications No. 74690/1992, No. 238036/1992 and No. 38878/1993. Further, many cyanine type dyes have recently been proposed, such as those disclosed in Japanese Unexamined Patent Publications No. 336086/1994, No. 76169/1995, No. 125441/1995, No. 262604/1995 and No. 266705/1995. However, these proposals are nothing more than mere application of the knowledge of CD-R at a wavelength of 780 nm to a shorter wavelength use, and they do not give any specific requirements to realize high recording density by making small sizes of written marks, which is a merit in shortening the operation wavelength.
Further, with respect to the shape of a groove, Japanese Unexamined Patent Publications No. 358331/1992, No. 198013/1993, No. 2771/1993, No. 109441/1992 and No. 22224/1991 are, for example, available, and they give some knowledge with respect to a wavelength around 780 nm, but they do not teach shortening of the wavelength.
In the recording at 780 nm, absorption and reduction in the film thickness due to the thermal decomposition of the dye and deformation due to softening of the substrate take place simultaneously. As the deformation of the record portion extends to the adjacent land portions or as the written mark width is larger, larger recording modulation amplitude can be obtained.
Further, as proposed in Japanese Unexamined Patent Publications No. 243508/1991, No. 50033/1995 and No. 44891/1995, a method has been developed wherein a super saturated absorbing layer is laminated, so that the beam diameter of a reproducing beam itself may be reduced by providing apertures, whereby an adequate degree of modulation can be obtained even with a very small record portion, and a resolution exceeding the spatial frequency limit represented by .lambda./(2NA) can be realized. However, in this method, fairly large absorption is required, and it is therefore difficult to obtain sufficient high reflectivity constantly.
In the above described prior art, a large recording modulation amplitude is obtained by both the decomposition of the dye and the deformation of the substrate. However, in a case where the recording density is to be increased at the same time as shortening the wavelength, for example, by reducing the track pitch to a level of less than 1 .mu.m, the deformation of the record portion tends to be too large and brings about a serious problem of crosstalk. This brings about a serious problem especially when recording is carried out in a groove. Further, reproduction is carried out at a wavelength shorter than the wavelength for CD-R and yet by means of a laser focusing lens having a large number of apertures (NA). Accordingly, as compared with 780 nm-NA0.5, in the case of 640 nm-NA0.6, the degree of focusing of the reproducing laser beam increases by 1.46 times. Consequently, the temperature rise and light intensity due to irradiation with the reproducing beam are higher when shorter wavelength laser pick up (drive) is used, so the dye which has high reproducibility is especially required.
Further, according to a conventional groove design, it is common to form written marks larger than the beam diameter at the time of using a laser head having a large number of apertures (NA) for high densification, whereby it has been difficult to obtain high reflective contrast between the written and unwritten portions.